Piston compressor

ABSTRACT

A gasket is provided between a cylinder block and a valve plate. By providing a through hole at a position near the center of the gasket, bending moment acting on the cylinder block is reduced, and hence deformation of the cylinder block is restrained. As a result, reciprocating motion of a piston and rotational motion of a rotary valve are performed smoothly.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a piston compressor for avehicular air conditioner and, more particularly, to a technology forrestraining deformation of a cylinder block.

[0002] For example, Japanese Laid-Open Patent Publication No. 8-19160discloses a gasket 101 as shown in FIG. 13. The gasket 101 is used for apiston compressor for a vehicular air conditioner.

[0003] The gasket 101 is formed with a plurality of through holes 103that substantially coincide with opening edges of cylinder bores 102each containing a piston, a plurality of insertion holes 105 throughwhich through bolts 104 are inserted, and a center hole 106 throughwhich a drive shaft is inserted. As a piston compressor provided withthis gasket 101, a piston compressor is known in which as shown in apartially enlarged cross-sectional view of FIG. 14, a front housingmember 108 is joined to a front end face (left-hand side in the figure)of a cylinder block 107, a rear housing member 110 is joined to a rearend face (right-hand side in the figure) thereof via a valve plate 109,and these three elements are fastened to each other by the through bolts104. In this piston compressor, the gasket 101 is interposed between thecylinder block 107 and the valve plate 109. As shown in FIG. 15, thecylinder block 107 is formed with the cylinder bores 102 and anaccommodation chamber 111 for accommodating a rotary valve for suckingrefrigerant gas.

[0004] In the piston compressor described in the above-describedPublication, when the through bolts 104 are tightened, the cylinderblock 107 is subjected to bending moment and is thus deformed.Specifically, as shown in FIG. 14, in the state in which the throughbolts 104 are tightened, on a joint surface between the cylinder block107 and the front housing member 108, a specific pressure f1 acts on thefront end face of the cylinder block 107 from the front housing member108. Also, on a joint surface between the cylinder block 107 and a sealsurface of the gasket 101, a specific pressure f2 acts on the rear endface of the cylinder block 107 from the valve plate 109.

[0005] Taking one arbitrary point on the front end face of the cylinderblock 107, on which the specific pressure f1 acts, as action point P1,and taking one arbitrary point on the rear end face of the cylinderblock 107, on which the specific pressure f2 acts, as action point P2,bending moment M acts around the center P3 of straight line H connectingP1 and P2. By this bending moment M, a force Fm in a radial direction ofthe gasket 101 is applied to both of the action points P1 and P2, bywhich the cylinder block 107 is deformed as indicated by two-dot chainlines shown in FIG. 15. As a result, there is a fear that smoothreciprocating motion of the piston is hindered by this deformation.

[0006] Also, in a case where the accommodation chamber 111 for therotary valve is formed in the cylinder block 107 as shown in FIG. 15,the accommodation chamber 111 is easily deformed because the rigidity ofthe cylinder block 107 is low. Therefore, smooth rotation of the rotaryvalve can be hindered.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide a pistoncompressor in which bending moment acting on a cylinder block is reducedto restrain deformation of the cylinder block, and the motion of apiston and a rotary valve is performed-smoothly to enhance thedurability of the piston compressor.

[0008] To achieve the foregoing and other objectives and in accordancewith the purpose of the present invention, a piston compressor having acylinder block, a front housing member, a rear housing member, a throughbolt, a plurality of pistons, a drive shaft, and a gasket is provided.The cylinder block has a plurality of cylinder bores. The cylinder blockhas two end faces at which the cylinder bores open. The front housingmember is secured to one of the end faces of the cylinder block. Therear housing member is secured to the other one of the end faces of thecylinder block with a valve plate in between. The through bolt fastensthe cylinder block, the rear housing member, and the front housing. Eachpiston is accommodated and reciprocates in one of the cylinder bores.The drive shaft drives the pistons, and is rotatably supported by thecylinder block. Reciprocation of the pistons compress and dischargerefrigerant gas. The gasket is located between the cylinder block andthe valve plate. The gasket has a center hole and a plurality of boreholes. Each bore hole is aligned with one of the cylinder bores. A firstthrough hole is formed in the gasket to reduce bending moment generatedin the cylinder block when the through bolt is fastened. The firstthrough hole is located between an adjacent pair of the bore holes andin an imaginary circle. The center of the imaginary circle coincideswith the center of the bore hole, and the radius of the imaginary circleis a first radius. The first radius is the distance from the center oftho gasket to the center of one of the bore holes.

[0009] Other aspects and advantages of the invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention, together with objects and advantages thereof, maybest be understood by reference to the following description of thepresently preferred embodiments together with the accompanying drawingsin which:

[0011]FIG. 1 is a cross-sectional view of a piston compressor inaccordance with a first embodiment of the present invention;

[0012]FIG. 2 is a partially cross-sectional view of the compressor shownin FIG. 1;

[0013]FIG. 3 is a front view of a gasket provided in the compressorshown in FIG. 1;

[0014]FIG. 4 is a front view of a conventional gasket used forexplanation of the first embodiment;

[0015]FIG. 5 is a graph showing a relationship between circumferentiallengths of seal portions necessary for function and distances from thegasket center in a gasket;

[0016]FIG. 6 is a graph showing a relationship between circumferentiallength of seal portions unnecessary for function and distances from thegasket center in a gasket;

[0017]FIG. 7 is a graph showing a total change amount of bending momentgenerated in a cylinder block;

[0018]FIG. 8 is a cross-sectional view of a piston compressor inaccordance with a second embodiment;

[0019]FIG. 9 is a front view of a gasket provided in the compressorshown in FIG. 8;

[0020]FIG. 10 is a front view of a conventional gasket used forexplanation of a second embodiment;

[0021]FIG. 11 is a front view of a gasket in a modified embodiment;

[0022]FIG. 12 is a front view of a gasket in another modifiedembodiment;

[0023]FIG. 13 is a front view of a prior art gasket;

[0024]FIG. 14 is a partially cross-sectional view of a prior art pistoncompressor; and

[0025]FIG. 15 is a partially cross-sectional view of a prior art pistoncompressor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] A first embodiment of the present invention will now be describedin detail with reference to FIGS. 1 to 7.

[0027] In a variable displacement piston compressor in accordance withthe first embodiment of the present invention, as shown in FIG. 1, afront housing member 3 is joined to a front end face of a cylinder block1 via a gasket 2, and a crank chamber 4 serving as a control chamber isdefined on the inside thereof. Also, a rear housing member 6 is joinedto a rear end face of the cylinder block 1 via a valve plate 5, and adischarge chamber 1 arid a suction chamber 8 are defined on the insidethereof. Between the cylinder block 1 and the valve plate 5 isinterposed a gasket 9, and between the valve plate 5 and the rearhousing member 6 are interposed a discharge valve forming plate 10formed integrally with a discharge valve and a retainer forming plate 11for forming a retainer. The cylinder block 1, the front housing member3, and the rear housing member 6 are fastened by through bolts 12, notshown in FIG. 1.

[0028] In shaft holes formed in central portions of the cylinder blockland the front housing member 3, a drive shaft 13 is rotatably supportedby radial bearings 14 a and 14 b. In a front end portion of the driveshaft 13 is provided a shaft seal device 15. In the crank chamber 4, alug plate 16 is fixed to the drive shaft 13 so as to be integrallyrotatable, and a swash plate 17 serving as a cam plate is disposed in astate in which the drive shaft 13 is inserted through a through holeformed in the swash plate 17. A hinge mechanism 18 is interposed betweenthe lug plate 16 and the swash plate 17. The swash plate 17 can berotated in synchronism with the lug plate 16 and the drive shaft 13 by ahinge connection between the swash plate 17 and the lug plate 16 via thehinge mechanism 18 and the support of the drive shaft 13, and also canbe tilted with respect to the drive shaft 13 while sliding in an axialdirection of the drive shaft 13.

[0029] A plurality of cylinder bores 19 arranged in a circumferentialdirection in the cylinder block 1 each contain a piston 20 capable ofreciprocating. Between each piston 20 and the valve plate 5, acompression chamber 21 whose volume is changed according toreciprocating motion of the piston 20 is defined. Each piston 20 isengaged with a peripheral edge portion of the swash plate 17 via a pairof shoe 22. Therefore, rotational motion of the swash plate 17 performedvia the lug plate 16 and the hinge mechanism 18, which is caused byrotation of the drive shaft 13, is converted to reciprocating motion ofthe pistons 20 performed via the shoes 22. The lug plate 16, the swashplate 17, the hinge mechanism 18, and the shoes 22 constitute a crankmechanism that converts the rotational motion 1 of the drive shaft 13 tocompressive motion for compressing refrigerant gas in the compressionchamber 21.

[0030] A rotary valve accommodating chamber 23 is formed in the cylinderblock 1, and in the rotary valve accommodating chamber 23, a rotaryvalve 24 is connected to the drive shaft 13 via a coupling 25 so as tobe rotatable in synchronism with the drive shaft 13. In the rotary valve24, a suction passage 26 that always communicates with the suctionchamber 8 is formed, and an outlet 27 of the suction passage 26 is openin an outer peripheral surface of the rotary valve 24. In the cylinderblock 1, communication holes 28 are formed. Each communication holecorresponds to one of the compression chambers 21 and allows the outlet27 of the rotary valve 24 to communicate with the correspondingcompression chamber 21.

[0031] When the drive shaft 13 of the compressor is rotated by enginepower, the swash plate 17 is rotated via the lug plate 16 and the hingemechanism 18, so that the pistons 20 are reciprocated in the cylinderbores 19 via the shoes 22. On a suction stroke of the piston 20, theoutlet 27 of the rotary valve 24 is connected to each communication hole28, so that the refrigerant gas in tho suction chamber 8 is sucked intoeach compression chamber 21 through the suction passage 26. Further,when each piston 20 takes compression stroke and discharge strokes, thecorresponding communication hole 28 is closed by an outer peripheralsurface of the rotary valve 24, so that the refrigerant gas in thecompression chamber 21 pushes away the discharge valve and is dischargedto the discharge chamber 7.

[0032] Next, an essential point of the present invention will bedescribed in detail. First, forces acting on the cylinder block 1 inthis embodiment are shown in FIG. 2. In a state in which the throughbolts 12 are tightened, on a joint surface between the cylinder block 1and the front housing member 3, a specific pressure f1 acts on a frontend face of the cylinder block 1 from the front housing member 3. Also,on a joint surface between the cylinder block 1 and a seal surface ofthe gasket 9, a specific pressure f2 acts on a rear end face of thecylinder block 1 from the gasket 9.

[0033] Taking one arbitrary point on the front end face of the cylinderblock 1, on which the specific pressure f1 acts, as action point P1, andtaking one arbitrary point on the rear end face of the cylinder block 1,on which the specific pressure f2 acts, as action point P2, bendingmoment M acts around the center P3 of straight line H connecting P1 andP2. When the shortest distance between both of the action points P1 andP2 in a radial direction of the gasket 9 is taken as D1, the shortestdistance therebetween in the axial direction of the through bolt 12 istaken as D2, and a radial force generated at both of the action pointsP1 and P2 by the bending moment M is taken as Fm, the bending moment Mis obtained by the following formulae:

Fm=f 2·(D 1/D 2)  (1)

M=Fm·D 2=f 2·D 1  (2)

[0034] From these two formulae, it is found that the force Fm and thebending moment M increase as the specific pressure f2 acting on the rearend face of the cylinder block 1 from the gasket 9 increases, or as theaction point P2 is closer to the center of the gasket 9.

[0035] The gasket 9 in this embodiment is shown in FIG. 3. The gasket 9is formed of a rigid base consisting of an iron-base metallic sheet andan elastic layer having sealing ability, such as rubber, with which bothsurfaces of the base are coated. Also, the gasket 9 has a plurality of(six in this embodiment) bore holes 29 that substantially coincide withthe opening edges of the cylinder bores 19 and a plurality of (six inthis embodiment) bolt holes 30 through which the through bolts 12 areinserted. In a circle whose radius is a distance Rb from the center ofthe gasket 9 to the center of each bore hole 29, a through hole isformed which corresponds to a center hole 31 (in a circle indicated bydotted line in FIG. 3) in the conventional gasket and first throughholes 32 communicating with each other. Between a circle having a radiusof a distance Rb from the center of the gasket 9 and a circle having aradius Rc from the center of the gasket 9, second through holes 33 areformed. As is apparent from FIG. 2, in the range in which the firstthrough holes 32 and the second through holes 33 are provided, thespecific pressure f2 does not act on the cylinder block 1, so thatbending moment is not generated. Because the bending moment is larger ata position closer to the center of the gasket 9, the provision of thethrough holes 32 and 33 can reduce the bending moment.

[0036] The meaning of the radius Rc and a method for determining thesame will be explained with reference to FIGS. 4 to 7. FIG. 4 shows aconventional gasket 34 formed with bore holes 29, bolt holes 30, and acenter hole 31. In FIG. 4, solid line hatched portions are seal portionsthat are necessary for function of sealing the bore holes 29, the boltholes 30, and the interior of the compressor. That is to say, in thegasket 34, the range excluding the solid line hatched portions, the boreholes 29, the bolt holes 30, and the center hole 31 (dotted line hatchedportions in FIG. 4) indicates portions that are unnecessary for thefunction of the gasket. The length of seal portions that are necessaryfor the function on the circumference of a circle whose radius is acertain distance x from the center O of the gasket 34 and the length ofseal portions that are unnecessary for the function on the circumferenceof a circle whose radius is a certain distance x from the center O arerepresented by graphs of FIGS. 5 and 6, respectively. Rg indicates theradius of the gasket 34. Here, a complement is given to the descriptionof “the length of seal portions on the circumference of a circle whoseradius is a certain distance x from the center O of the gasket 34”. Forexample, when the length of seal portions that are necessary for thefunction on the circumference of a circle whose radius is a distance Afrom the center O is taken as La, and the length of seal portions thatare unnecessary for the function thereon is taken as Lb, as is apparentfrom FIG. 4, La and Lb are expressed as

La=L 1+L 3+L 5+L 7+L 9+L 11

Lb=L 2+L 4+L 6+L 8+L 10+L 12

[0037] From FIGS. 5 and 6, an area S of seal portions of the gasket 34is calculated by the following formula (3).

S=∫ _(O) ^(Rg) f(x)dx+∫ _(O) ^(Rb) g(x)dx+∫ _(Rb) ^(Rg) h(x)dx  (3)

[0038] In the above formula (3), the function f(x) is a function for thegraph of FIG. 5, the function g(x) is a function for the range of 0≦x≦Rbin the graph of FIG. 6, and the function h(x) is a function for therange of Rb≦x≦Rg in the graph of FIG. G.

[0039] Further, when the total pressure applied to the whole of a sealsurface of the gasket 34 at the time of tightening of the through bolts12 is taken as F, the specific pressure f2 per unit area of the sealsurface is expressed as

f 2=F/S

[0040] The total pressure F depends on the tightening force of bolt, andthe shape, rigidity of the cylinder block and rear housing member, andit is thought that the total pressure F in this embodiment is equivalentto that of the conventional compressor.

[0041] Next, it is assumed that through holes with a minute width Δx areprovided in the portions that are unnecessary for the function (dottedline hatched portions in FIG. 4) on the circumference whose radius is acertain distance x from the center O. An area S(x) of seal portions atthis time is calculated by the following two formulae.$\left\{ \begin{matrix}{{S(x)} = {S - {\int_{x}^{x + {\Delta \quad x}}{{g(x)}\quad {x}}}}} & {\left( {{{when}\quad 0} \leq x \leq {Rb}} \right)\quad (4)} \\{{S(x)} = {S - {\int_{x}^{x + {\Delta \quad x}}{{h(x)}\quad {x}}}}} & {\left( {{{when}\quad {Rb}} \leq x \leq {Rg}} \right)\quad (5)}\end{matrix} \right.$

[0042] When the increase in specific pressure at the time when thethrough holes with a minute width Δx are provided is taken as Δf2, Δf2can be expressed as Δf2=F/S(x)−F/S using the above-described formulae(4) and (5).

[0043] Therefore, taking the increase in bending moment as ΔM1, ΔM1 canbe expressed by the following formula (6) using the above-describedformula (1) and the above-described Δf2.

ΔM 1=∫_(O) ^(Rg)(Δf 2·x)dx  (6)

[0044] Also, taking the decrease in bending moment due to the provisionof through holes as ΔM2, from the above-described formula (2), ΔM2 isexpressed as

ΔM 2=f 2·x  (7)

[0045] Therefore, when the total change amount of bending moment at thetime when the through holes are provided in the portions that areunnecessary for the function on the circumference whose radius is acertain distance x from the center O is taken as ΔM (=ΔM2−ΔM1), ΔM isexpressed by a graph shown in FIG. 7 using Formulae (6) and (7). Rc isdefined as a distance of a point at which ΔM1=ΔM2 (≠0) from the centerO. In FIG. 7, Rc denotes a point at which ΔM=0 (excluding a case whereΔM1=ΔM2=0 is satisfied).

[0046]FIG. 7 means that if through holes arc formed in a circle with theradius Rc from the center O, since the decrease in bending moment due tothe through holes is larger than the increase in bending moment due toincreased specific pressure, the total bending moment can be decreased.

[0047] In this embodiment, a seal portion for sealing the compressorinternally and externally is provided in an outer peripheral portion ofthe gasket 9. As is apparent from FIG. 2, bending moment is notgenerated on a joint surface 35 between the cylinder block 1 and thegasket 9, which faces a joint surface between the cylinder block 1 andthe front housing member 3 in the axial direction of the drive shaft 13.Therefore, it is desirable that the gasket 9 be formed with a sealsurface in the range of the joint surface 35 so as to decrease thespecific pressure Δf2 as much as possible.

[0048] By this embodiment, the bending moment acting on the cylinderblock 1 is reduced, and hence the deformation of the cylinder block 1 isrestrained. As a result, the deformation of the cylinder bore 19 isrestrained, and hence the reciprocating motion of the piston 20 is madesmooth. Also, the deformation of the rotary valve accommodating chamber23 for the rotary valve 24 is restrained, and hence the rotationalmotion of the rotary valve 24 is made smooth. Further, the specificpressure of gasket is increased by reducing the seal surface, so thatthe sealing ability of gasket is improved, or sufficient sealing abilityof gasket is secured even if the tightening force of bolts is decreasedas compared with the conventional compressor. Therefore, the deformationof the cylinder block 1 can further be restrained by the decrease inbolt tightening force, and hence the durability of compressor isenhanced.

[0049] Next, a second embodiment will be described with reference toFIGS. 8 to 10. In the second embodiment, only points different from thefirst embodiment shown in FIGS. 1 to 7 will be explained. Also, the samereference numerals will be applied to the same or equivalent elements,and the explanation of the elements will be omitted.

[0050]FIG. 8 shows a five-cylinder compressor. In this compressor, therotary valve 24 and the rotary valve accommodating chamber 23 are notused as a suction structure for refrigerant gas, and instead a suctionvalve forming plate 36 is interposed between the cylinder block 1 andthe valve plate 5, and a gasket 37 is interposed between the suctionvalve forming plate 36 and the cylinder block 1. On the suction strokeof each piston 20, a corresponding suction valve is opened, and arefrigerant gas passes through a corresponding suction hole formed inthe valve plate 5 and is sucked into the compression chamber 21.Further, when the piston 20 takes compression and discharge strokes, thesuction valve is closed, and the suction hole is closed and therefrigerant gas in the compression chamber 21 pushes away the dischargevalve and is discharged to the discharge chamber 7.

[0051] As shown in FIG. 9, in the gasket 37 used in this embodiment, onethrough hole 38 is formed in a state in which the center hole 31 (in acircle indicated by dotted line in FIG. 9), the first through holes, andthe second through holes communicate with each other. In the pistoncompressor of this embodiment, the number of cylinders is decreased tofive as compared with the above-described first embodiment. FIG. 10shows a conventional gasket 39 used for a five-cylinder pistoncompressor. In FIG. 10, hatched portions are seal portions that arenecessary for function of sealing the bore holes 29, the bolt holes 30,and the interior of the compressor. As is apparent from FIG. 10, in thegasket 39, seal portions that are unnecessary for the function arepresent even between the adjacent bore holes 29. Therefore, as in thegasket 37 of this embodiment, it is possible to form the integralthrough hole 38 by allowing the center hole 31, the first through holes,and the second through holes to communicate with each other. Thereby,the bending moment is reduced, and resultantly the deformation of thecylinder block 1 is restrained. Also, by forming the integral throughhole 38 in this manner, a mold necessary for manufacturing the gasket 37is formed easily, and the life of mold is extended, which also achievesan effect of reducing the manufacturing cost.

[0052] It should be apparent to those skilled in the art that thepresent invention may be embodied in many other specific forms withoutdeparting from the spirit or scope of the invention. Particularly, itshould be understood that the invention may be embodied in the followingforms.

[0053] As shown in FIGS. 11 and 12, the center hole 31 and the firstthrough holes 32 may be separated from each other.

[0054] In these examples as well, the deformation of the cylinder blockis restrained by reducing bending moment, and hence the motion of thepiston and rotary valve is made smooth, by which the durability of thepiston compressor is enhanced.

[0055] The present examples and embodiments are to be considered asillustrative and not restrictive and the invention is not to be limitedto the details given heroin, but may be modified within the scope aridequivalence of the appended claims.

1. A piston compressor comprising: a cylinder block having a plurality of cylinder bores, wherein the cylinder block has two end faces at which the cylinder bores open; a front housing member, which is secured to one of the end faces of the cylinder block; a rear housing member, which is secured to the other one of the end faces of the cylinder block with a valve plate in between; a through bolt for fastening the cylinder block, the rear housing member, and the front housing; a plurality of pistons, each of which is accommodated and reciprocates in one of the cylinder bores; a drive shaft for driving the pistons, wherein the drive shaft is rotatably supported by the cylinder block, wherein reciprocation of the pistons compress and discharge refrigerant gas; and a gasket located between the cylinder block and the valve plate, wherein the gasket has a center hole and a plurality of bore holes, each bore hole being aligned with one of the cylinder bores, wherein a first through hole in formed in the gasket to reduce bending moment generated in the cylinder block when the through bolt is fastened, wherein the first through hole is located between an adjacent pair of the bore holes and in an imaginary circle, the center of the imaginary circle coinciding with the center of the bore hole and the radius of the imaginary circle being a first radius, and wherein the first radius is the distance from the center of the gasket to the center of one of the bore holes.
 2. The compressor according to claim 1, wherein the imaginary circle is a first imaginary circle, wherein a second imaginary circle having a second radius is assumed to exist about the center of the gasket, the second radius being greater than the first radius by a predetermined value, wherein a second through hole is formed in the gasket to reduce bending moment generated when the through bolt is fastened, and wherein the second through hole is located in a portion of the gasket between the second imaginary circle and the first imaginary circle.
 3. The compressor according to claim 2, wherein the first through hole communicates with the second through hole.
 4. The compressor according to claim 1, wherein the imaginary circle is a first imaginary circle, wherein a second imaginary circle having a second radius Rc is assumed to exist about the center of the gasket, the second radius Rc is different from the first radius, wherein a second through hole is formed in the gasket to reduce bending moment generated in the cylinder block when the through bolt is fastened, and wherein the second through hole is located in a portion of the gasket between the second imaginary circle and the first imaginary circle, and wherein, with respect to a pressure applied to the cylinder bore by the gasket when the through bolt is fastened, if f denotes the pressure on the assumption that the gasket does not have the first and second through holes; Δf denotes the amount of increase of the pressure relative to the pressure f when a through hole is formed on the second imaginary circle of the gasket on the assumption that the gasket does not have the first and second through holes; and R denotes an arbitrary distance from the center of the gasket, the second radius Rc is determined such that f·Rc is equal to an integration value obtained by integrating Δf·R from the center of gasket over the range of the radius of the gasket.
 5. The compressor according to claim 4, wherein f·Rc represents a decrease amount of the bending moment when a through hole is formed on the second imaginary circle of the gasket on the assumption that the gasket does not have the first and second through holes, and wherein the integration value represents an increase amount of the bending moment when a through hole is formed on the second imaginary circle of the gasket on the assumption that the gasket does not have the first and second through holes.
 6. The compressor according to claim 4, wherein the first through hole communicates with the second through hole.
 7. The compressor according to claim 1, wherein the first through hole communicates with the center hole.
 8. The compressor according to claim 1, wherein a compression chamber is defined in each cylinder bore by the corresponding piston, wherein the compressor further comprising a suction pressure zone, the internal pressure of which is a suction pressure, and a rotary valve that rotates as the drive shaft rotates, and wherein the rotary valve has an introducing passage for successively introducing gas from the suction pressure zone to the compression chambers as the drive shaft rotates.
 9. The compressor according to claim 2, wherein the cylinder bores are provided about an axis of the cylinder block at equal angular intervals.
 10. The compressor according to claim 9, wherein the first through hole is one of a plurality of first through holes, the second through hole is one of a plurality of second through holes, wherein the first through holes are provided about the center of the gasket at equal angular intervals, and wherein each second through hole forms a pair with one of the first through holes.
 11. A piston compressor comprising: a cylinder block having a plurality of cylinder bores, wherein the cylinder block has two end faces at which the cylinder bores open; a front housing member, which is secured to one of the end faces of the cylinder block; a rear housing member, which is secured to the other one of the end faces of the cylinder block with a valve plate in between; a through bolt for fastening the cylinder block, the rear housing member, and the front housing; a plurality of pistons, each of which is accommodated and reciprocates in one of the cylinder bores; a drive shaft for driving the pistons, wherein the drive shaft is rotatably supported by the cylinder block, wherein reciprocation of the pistons compress and discharge refrigerant gas; and a gasket located between the cylinder block and the valve plate, wherein the gasket has a center hole and a plurality of bore holes, each bore hole being aligned with one of the cylinder bores, wherein the cylinder bores are provided about an axis of the cylinder block at equal angular intervals, wherein a through hole is formed in the gasket to reduce bending moment generated in the cylinder block when the through bolt is fastened, wherein the through hole is located in an imaginary circle, the radius of the imaginary circle being a first radius, wherein the first radius is the distance from the center of the gasket to the center of one of the bore holes, and wherein the through hole has a section that is located between an adjacent pair of the cylinder bores.
 12. The compressor according to claim 11, wherein the through hole is a first through hole, and the imaginary circle is a first imaginary circle, wherein a second imaginary circle having a second radius is assumed to exist about the center the gasket, the second radius being greater than the first radius by a predetermined value, wherein a second through hole is formed in the gasket to reduce bending moment generated when the through bolt is fastened, and wherein the second through hole is located in a portion of the gasket between the second imaginary circle and the first imaginary circle.
 13. The compressor according to claim 12, wherein the first through hole communicates with the second through hole.
 14. The compressor according to claim 11, wherein the through hole is a first through hole, and the imaginary circle is a first imaginary circle, wherein a second imaginary circle having a second radius Rc is assumed to exist about the center of the gasket, the second radius Rc is different from the first radius, wherein a second through hole is formed in the gasket to reduce bending moment generated in the cylinder block when the through bolt is fastened, and wherein the second through hole is located in a portion of the gasket between the second imaginary circle and the first imaginary circle, and wherein, with respect to a pressure applied to the cylinder bore by the gasket when the through bolt is fastened, if f denotes the pressure on the assumption that the gasket does not have the first and second through holes; Δf denotes the amount of increase of the pressure relative to the pressure f when a through hole is formed on the second imaginary circle of the gasket on the assumption that the gasket does not have the first and second through holes; and R denotes an arbitrary distance from the center of the gasket, the second radius Rc is determined such that f·Rc is equal to an integration value obtained by integrating Δf·R from the center of gasket over the range of the radius of the gasket.
 15. The compressor according to claim 14, wherein f·Rc represents a decrease amount of the bending moment when a through hole is formed on the second imaginary circle of the gasket on the assumption that the gasket does not have the first and second through holes, and wherein the integration value represents an increase amount of the bending moment when a through hole is formed on the second imaginary circle of the gasket on the assumption that the gasket does not have the first and second through holes.
 16. The compressor according to claim 14, wherein the first through hole communicates with the second through hole.
 17. The compressor according to claim 11, wherein the first through hole communicates with the center hole.
 18. The compressor according to claim 11, wherein a compression chamber is defined in each cylinder bore by the corresponding piston, wherein the compressor further comprising a suction pressure zone, the internal pressure of which is a suction pressure, and a rotary valve that rotates as the drive shaft rotates, and wherein the rotary valve has an introducing passage for successively introducing gas from the suction pressure zone to the compression chambers as the drive shaft rotates.
 19. The compressor according to claim 12, wherein the first through hole is one of a plurality of first through holes, the second through hole is one of a plurality of second through holes, wherein the first through holes are provided about the center of the gasket at equal angular intervals, and wherein each second through hole forms a pair with one of the first through holes. 